Among conventional control methods of this kind are one described in Japanese Patent Publication No. 20817/1990, in which the starting and ending points of an engine torque down control to reduce shift shocks are determined based on the engine rotation speed at the start of gear shift changing; one described in Japanese Patent Publication No. 5688/1993, in which the starting and ending points of the engine torque down control to reduce shift shocks are determined based on the input/output rotation speed ratio, i.e., the ratio of the input shaft rotation speed (turbine rotation speed) and the output shaft rotation speed (called a vehicle speed signal); and one described in Japanese Patent Publication No. 81658/1992, which determines the starting point of the engine torque down control by the first method described above and the ending point of this control by the second method described above.
The engine torque down control of this kind of conventional control, as described in Japanese Patent Publication No. 7213/1993, generally switches from a normal characteristic data memory of the engine control device to a gear shift changing characteristic data memory during the above-mentioned period, and is required to search, through prestored maps, for the control timing and control value for every gear position and engine load, and to control them.
FIG. 3 is a time chart that explains the shift shock reduction method using the above-mentioned conventional technique. This method uses an ignition timing as the control value for engine torque down. Control timings t1, t2 are determined to be the times when the input/output rotation speed ratio exceeds prestored set values S1, S2. In this control period between t1 and t2, a prestored control value, i.e., an ignition timing retard angle .DELTA..theta., is read out and added to a base ignition timing.
Hence, the corrected control value in this correction control period is constant. When the output shaft torque in this control period is almost flat as shown, the retard control mentioned above can reduce torque fluctuation significantly. In other words, a so-called shift shock reduction effect is produced. The actual torque waveforms, however, vary greatly during that control period, making it impossible in many cases for a constant correction value to produce a sufficient correction effect.
The set values S1, S2 and ignition timing retard angle .DELTA..theta. need to be made optimum by tuning at the development stage, requiring a very long time. Even if they can be made optimum, change with time and change in environment may render the set values thus determined inadequate for control, making it difficult to completely reducing the shift shocks.